Radiation-sensitive silver halide grains can be prepared by a variety of techniques. One common approach is a batch preparation technique commonly referred to as single-jet (or single-run) precipitation. According to this procedure, a silver salt solution is run into a reaction vessel containing a halide salt solution and a dispersing medium, usually gelatin. The first halide ions already in the reaction vessel react with silver salt to form silver halide grain nuclei. As additional silver salt solution is introduced, more silver halide is formed. Some of this silver halide forms additional nuclei while the remaining silver halide is concurrently deposited on the existing silver halide grain nuclei. Single-jet precipitation is discussed by T.H. James, "The Theory of the Photographic Process," 4th Ed., MacMillan, 1977, Chapter 3 and is specifically illustrated by Trivelli and Smith "The Photographic Journal," Vol. LXXIX, May 1939, pp. 300-338.
Another approach to silver halide preparation is a batch approach commonly referred to as a double-jet (or double-run) precipitation. According to this approach, a silver salt solution and a halide salt solution are concurrently fed into a reaction vessel containing a dispersing medium, commonly gelatin dissolved in water. Precipitation of silver halide grains preferably occurs in two distinct stages. In a first, nucleation stage, initial silver halide grain formation occurs. This is followed by a second, growth stage in which additional silver halide, formed as a reaction product, precipitates onto the initially formed silver halide grains. This results in growth of the silver halide grains. Batch double-jet precipitation typically takes place under conditions of rapid stirring of reactants in which the volume within the reaction vessel continuously increases during the silver halide precipitation make or run.
Continuous double-jet precipitation procedures are known, as illustrated by British Patent No. 1,302,405, U.S. Pat. Nos. 3,801,326 to Claes, and 4,046,576 to Terwilliger et al.
U.S. Pat. No. 3,790,386 to Posse et al. is directed to a variant form of continuous double-jet precipitation in which the silver halide emulsion is continuously withdrawn from a constant volume reaction vessel and fed to a separate ripening vessel which is at least 10 times the volume of the reaction vessel. No provision is made for the removal of soluble salts or dispersing medium from the silver halide emulsions produced. As a result, the total volume of the emulsion in the reaction and ripening vessels increases in direct proportion to the volume of salt solution added. Similar continuous double-jet precipitation arrangements are disclosed by U.S. Pat. Nos. 3,897,935 to Forster et al., 4,147,551 to Finnicum et al., and 4,171,224 to Verhille et al.
The purification and concentration of silver halide emulsions by ultrafiltration is known in the art. Such techniques are illustrated by "Research Disclosure," Vol. 102, October 1972, Item 10208, and Vol. 131, March 1975, Item 13122. "Research Disclosure" is published by Industrial Opportunities, Ltd., Homewell, Havant Hampshire, P09, IEF, U. K. It is also recognized that the soluble salts, such as alkali nitrate, formed as a by-product in precipitating silver halide, can be washed and removed by ultrafiltration while continuously adding makeup water to the emulsion. After the soluble salt content has been reduced to the desired level, it is taught to terminate the addition of makeup water and to reduce the liquid volume of the emulsion also by ultrafiltration. The liquid volume of the emulsion can also be reduced by ultrafiltration after addenda have been added and before coating.
Ultrafiltration is preferably accomplished by pumping emulsion from the reaction vessel into contact with a semipermeable membrane of the ultrafiltration module so that a pressure difference is established across the membrane. The membrane contains pores sized to permit passage of molecules below a particular molecular weight while retaining larger molecules and silver halide grains in the emulsion.
The membranes employed in ultrafiltration are typically anisotropic membranes which comprise a very thin layer of extremely fine pore texture supported upon a thicker porous structure. Useful membranes can be formed from a variety of polymeric materials, such as poly(vinyl chloride), poly(vinyl carboxylate) (e.g., poly(vinyl formate) and poly(vinyl acetate)), poly(vinyl alcohol), polysulfones, poly(vinyl ether), polyacrylamides and polymethacrylamides, polyimides, polyesters, polyfluoroalkylenes (e.g., polytetrafluoroethylene and polyvinylidene fluoride), and cellulosic polymers, such as, cellulose and cellulose esters (e.g., cellulose acetate, cellulose butyrate and cellulose acetate butyrate).
In U.S. Pat. No. 4,336,328 to Brown et al., the walls of the silver halide reaction vessel are formed from an ultrafiltration membrane which concentrates silver halide grains within the vessel, while permitting solvent and low molecular weight materials to permeate.
U.S. Pat. Nos. 4,334,012 to Mignot ("Mignot") and 4,758,505 to Hoffmann ("Hoffmann") disclose ultrafiltration modules which are in a loop through which photographic emulsions in the silver halide reaction vessel circulate. In Hoffmann, the ultrafiltration module is provided with a preliminary filter (e.g., a sieve). Mignot's system utilizes a single ultrafiltration module or a plurality of such modules placed in series to permit the dispersing medium from the first module to be fed to the next. The concentrate from each of Mignot's serially-arranged ultrafiltration modules is returned to the reaction vessel. Hoffmann mentions using either ultrafiltration modules in a reaction vessel or tubular ultrafiltration modules without suggesting any preference amongst these alternatives.
Once concentrated and washed, the emulsion must be finished by adding chemical and optical sensitizers so that the emulsion will react to particular intensities and wavelengths of radiation. The finished silver halide emulsion can then be applied to photographic film, paper, or plates as one of a plurality of separate and distinct layers. Such coatings are conventionally applied by bead coating or by curtain coating. Examples of both these coating techniques are disclosed in U.S. Pat. No. 4,287,240 to O'Connor. Such coating procedures require that the emulsion be concentrated to a high viscosity level. This prevents inter-layer mixing in the coated film or paper, which is a particularly significant problem in the lower coating layers of such articles. It is especially important for photographic emulsions to have a high viscosity in curtain coating operations to ensure that the curtain remains stable and hangs properly. A high viscosity is also desirable so that both the volume of emulsion stored between ultrafiltration and coating as well as the amount of water to be removed after coating are reduced.
Unfortunately, the use of conventional spiral wound ultrafiltration modules alone cannot concentrate photographic emulsions on a commercial scale to a viscosity suitable for curtain coating. This is because the construction of the spiral wound ultrafiltration module limits the maximum concentrate viscosity to typically 20-40 centipoise, a value dependent strongly on the design of the module. Plate and frame ultrafiltration modules can concentrate to viscosities above 100 centipoise, but with long process times. This occurs because plate and frame ultrafiltration modules have a much smaller membrane area that contacts a given volume of liquid than do spiral wound ultrafiltration modules. As a result, neither a spiral wound ultrafiltration module nor a plate and frame ultrafiltration module can practically produce high viscosity emulsions under commercial scale conditions.